The present invention relates to an electromagnetic connecting device including a field core with an annular coil storage groove.
An electromagnetic connecting device includes a rotor that receives power transmitted from a driving source such as an engine or a motor and rotates, an armature facing the friction surface of the rotor across an air gap, a field core inserted into the core storage groove of the rotor, an electromagnetic coil stored in the coil storage groove of the field core, and the like.
Among electromagnetic connecting devices of this type, there is an electromagnetic clutch that switches between transmission and cutoff of power to a compressor for a car air conditioner. The electromagnetic clutch is mounted on the front housing side of the compressor. The rotor of the electromagnetic clutch is rotatably supported by the cylindrical nose portion of the front housing via a bearing. The rotating shaft of the compressor is inserted into the axial portion of the nose portion. The armature is supported, via a leaf spring, by an armature hub integrally rotatably key-fitted in an axial end of the rotating shaft. The field core is formed from an inner cylindrical portion, an outer cylindrical portion, and an annular bottom portion that connects ends of these members on one side to each other. In the field core, an annular coil storage groove surrounded by the inner cylindrical portion, the outer cylindrical portion, and the bottom portion is formed. The inner peripheral surface of the inner cylindrical portion and the outer peripheral surface of the outer cylindrical portion are flat cylindrical surfaces formed on the same axis as the center line of the field core.
A large field core is formed by cutting a columnar blank made of a low-carbon steel material for mechanical structure or a material similar to that into an annular member with a U-shaped section. General field cores, including large field cores, are formed by drawing such as hot forging or cold forging, cutting, and the like. That is, a field core is formed as an annular member with a U-shaped section by setting a blank made of a plate material such as a cold rolled steel sheet or a hot rolled steel sheet in a mold of press working (drawing such as hot forging or cold forging), plastically deforming the blank into an almost U-shaped section using a molding press die so as to work it into a semi-finished product, and then performing a step such as cutting.
The electromagnetic coil stored in the field core is generally formed by a copper wire. As a wire used to form the electromagnetic coil, an aluminum wire is sometimes used to reduce the weight and manufacturing cost of the field core, as described in, for example, Japanese Patent Laid-Open No. 2009-243678 (literature 1).
However, the conductivity of the aluminum wire is lower than that of the copper wire. For this reason, when using an aluminum wire, it is necessary to select an aluminum wire whose wire diameter is larger than that of a copper wire and make an ampere-turn AT (an attraction force acting on the armature by the magnetic flux) equal to that of the copper wire. Since the volume of an electromagnetic coil made by winding a thick aluminum wire multiple times increases as compared to a case in which a copper wire is used, the storage space of the coil storage groove to store the electromagnetic coil needs to be made large.
To implement this, an arrangement as shown in FIG. 7 can be considered. A field core 7 is formed by an inner cylindrical portion 9, an outer cylindrical portion 10, and a bottom portion 11, and includes a coil storage groove 12. An electromagnetic coil 8 is sealed by an insulating resin 13 in a state in which the electromagnetic coil 8 is inserted into the coil storage groove 12. In the field core 7, to make the capacity of the coil storage groove 12 large, the thickness of the bottom portion 11 is made small as compared to a case in which an electromagnetic coil using a copper wire is stored. The bottom surface of the bottom portion 11 in a case in which the electromagnetic coil using the copper wire is stored is indicated by an alternate long and two short dashed line L1 in FIG. 7.
However, when the bottom portion 11 of the field core 7 is formed thin, as shown in FIG. 7, a bottom surface 12a of the coil storage groove 12 is largely apart from end faces 4a and 6a of an inner cylindrical portion 4 and an outer cylindrical portion 6 of a rotor 5 to the side of a front housing 1. In this case, because of the small thickness of the bottom portion 11 of the field core 7, the flux density increases in bases 9a and 10a (ends close to the bottom portion 11) of the inner cylindrical portion 9 and the outer cylindrical portion 10 of the field core 7, and magnetic saturation occurs. That is, the flow of the magnetic flux between the field core 7 and the rotor 5 deteriorates. If such magnetic saturation occurs in the magnetic circuit, a magnetic attraction force for magnetically attracting the armature to the rotor 5 weakens.
Such a problem is solved by forming the inner cylindrical portion 4 and the outer cylindrical portion 6 of the rotor 5 long in the axial direction and reducing the gap between the distal ends of the cylindrical portions of the rotor 5 and the bottom portion 11 of the field core 7, as indicated by alternate long and two short dashed lines L2 and L3 in FIG. 7. However, if the inner cylindrical portion 4 and the outer cylindrical portion 6 of the rotor 5 become long in the axial direction, the weight of the rotor 5 increases. In addition, since the number of manufacturing steps in performing hot forging or the like increases, cost reduction of the electromagnetic clutch becomes difficult. Note that in FIG. 7, reference numeral 2 denotes a nose portion of the front housing 1; and 3, a bearing.